stylo: Don't keep around cached style after one-off style resolutions.

ports/geckolib/glue.rs

@@ -807,6 +807,8 @@ pub extern "C" fn Servo_ResolveStyle(element: RawGeckoElementBorrowed,
                                      consume: structs::ConsumeStyleBehavior,
                                      compute: structs::LazyComputeBehavior) -> ServoComputedValuesStrong
 {
+    use style::context::StyleContext;
+
     let element = GeckoElement(element);
     debug!("Servo_ResolveStyle: {:?}, consume={:?}, compute={:?}", element, consume, compute);
 
@@ -833,6 +835,9 @@ pub extern "C" fn Servo_ResolveStyle(element: RawGeckoElementBorrowed,
 
             recalc_style_at::<_, _, RecalcStyleOnly>(&context, &mut traversal_data, element, &mut data);
 
+            // We don't want to keep any cached style around after this one-off style resolution.
+            context.local_context().style_sharing_candidate_cache.borrow_mut().clear();
+
             // The element was either unstyled or needed restyle. If it was unstyled, it may have
             // additional unstyled children that subsequent traversals won't find now that the style
             // on this element is up-to-date. Mark dirty descendants in that case.